Refactor backend tasks to use cross-platform variables

Eliminate platform-specific command duplication in backend.yml by introducing GRADLE/TEST/FORMAT/CLEAN variables. Reorganize test, format, and format:check tasks into matrix patterns that explicitly test all flavor/security combinations. Add PowerShell script for reliable version extraction on Windows.

scripts/backend-version.ps1

@@ -0,0 +1,9 @@
+$ErrorActionPreference = 'Stop'
+
+$output = & "$PSScriptRoot/../gradlew.bat" printVersion --quiet 2>&1
+$lines = @($output | Where-Object { $_.ToString().Trim() })
+if ($lines.Count -eq 0) {
+    exit 0
+}
+
+Write-Output $lines[-1]